Vibration test machines (also referred to as “shakers”) have been used in the past for applying vibrational test loads to a test object to simulate the vibration experienced during actual use conditions. Such vibration test systems have application to the military, aeronautics, aerospace, satellite communications, automotive, and electronics industries, i.e., for providing nondestructive testing of components or devices eventually subjected to vibrational motions during use. One example is testing to simulate the vibration forces of a launch vehicle on a communications satellite. One critical vibration test for communication satellites, as an example, involves use of a single axis vibration test system for testing vibration effects in the vertical axis.
Single axis vibration test machines in the past have experienced certain undesired levels of cross-axis motion, which in turn produce inaccurate test results. The objective of such a vertical axis shaker is to move in the vertical axis only, and to avoid cross-axis motion by resisting any lateral translation motion and any rotational motions around any axis perpendicular to the line of motion. Stated another way, motion around a horizontal axis (pitch and roll motion) is to be resisted.
Conventional single axis shakers have not avoided the unwanted effects of cross-axis vibration. Conventional vertical axis vibration systems have been known to generate undesired cross-axis motions when the inherent mechanical resonances of the system are excited by the vibration applied to the vertical axis.
Commercially available shakers by themselves are often incapable of supporting and guiding large and/or heavy test items. The weight of many test items may exceed the allowable weight specified by the shaker manufacturer, and the center of gravity location may produce moment loads that exceed the allowable load on the shaker. During the vibration test, the dynamic behavior of the test item may produce forces that exceed the allowable values as specified by the shaker manufacturer.
In these situations, auxiliary equipment is required in order to support and guide the test article during the vibration test, and also to protect the shaker from excessive loads that would otherwise be induced by the behavior of the test article during the vibration test.
In one application involving a vertical axis shaker used for vibration testing of a communications satellite, the vertical axis test frequency range is from about 5 to about 200 Hz. Conventional vertical axis shakers and conventional auxiliary test article guidance equipment often have mechanical resonances within this frequency range.
An objective of the present invention is to reduce extraneous cross-axis motions in a vertical vibration test system that uses auxiliary guidance equipment by designing the system to be free of resonances within the range of frequencies to be tested. “System” means the complete mechanical assembly consisting of the shaker, the head expander, and the components that guide the head expander and resist any off-axis forces that may be produced by the test item during the vibration test.